Conventional production processes for converting petroleum feedstock into olefins typically involve molecular sieve catalysts. Molecular sieve catalysts generally contain molecular sieve particles that act as the catalyst component. An example of a molecular sieve that acts as a catalyst in converting oxygenates to olefins is a silicoaluminophosphate (SAPO) molecular sieve. Such molecular sieves contain a pore system, which is a network of uniform pores and empty cavities. These pores and cavities catch molecules that have a size equal to or less than the size of the pores and cavities, and repel molecules of a larger size. The active sites of the molecular sieves that have catalytic activity are generally located within the pores and cavities such that feed enters into the pores, contacts the active catalytic site, and is converted to product.
Molecular sieve catalysts can be characterized in terms of their activity and selectivity. The activity of a molecular sieve catalyst refers to the reaction rate for conversion of methanol (or another oxygenate) to olefin in the presence of the catalyst. The selectivity of molecular sieve catalyst refers to the type of olefins produced during the conversion reaction. For example, the prime olefin selectivity of a catalyst refers to the amount of ethylene and propylene produced relative to the total amount of olefin produced during a reaction.
One of the challenges in using molecular sieve catalysts is balancing the reactivity of the molecular sieve against the selectivity of the molecular sieve for producing desired olefins. Using conventional catalysts, it is possible to select a catalyst that provides the highest reaction rate for a given set of reaction conditions. A higher reaction rate can allow more feedstock to be processed in a reactor of fixed volume, or can reduce the required size of reactor needed to process a quantity of feedstock. However, catalysts with higher reaction rates typically also have lower selectivities for production of ethylene and propylene. Although more of the initial oxygenate feedstock is converted to a product, the percentage that is converted to the desired light olefin products is reduced. Catalysts with higher reactions rates also often produce increased amounts of coke. Coke is a carbon by-product of a conversion reaction that tends to deposit on the surface of the catalyst, leading to decreases in catalyst reactivity.
U.S. Patent Application Publication 2002/0165090 describes the preparation of SAPO molecular sieve catalysts that have intergrown phases. These intergrowth materials are composed of molecular sieve crystals that contain two separate frameworks, such as AEI and CHA.
U.S. Patent Application Publication 2003/0004384 describes a method for converting oxygenates to olefins. Either individual molecular sieves combined into one catalyst particle or mixtures of catalytic particles each containing a single molecular sieve can be used in the conversion reaction. Specifically described are two types of catalyst mixtures: one where each catalyst particles contains more than one type of SAPO molecular sieve, and one where each particle contains only one type of SAPO molecular sieve, but more than one type of SAPO molecular sieve catalyst particle is present.
What is needed is a catalyst or catalyst formulation that allows for improved conversion of an initial feedstock while minimizing the loss of reaction selectivity in producing desired olefins. The catalyst or catalyst formulation should be compatible with existing reaction systems.